Gu Ben-Yuan

Design of diffractive-phase axicon illuminated by a Gaussian-profile beam

The diffractive-phase axicon can convert the Gaussian-profile beam into axial uniform intensity distribution with long focal depth and high lateral resolution. Two types of phase-retardation functions for the nonuniform-illuminating axicon are derived in terms of the ray tracing and the geometrical law of energy conservation. Based on the general theory of the amplitude-phase retrieval in optical system and the iteration algorithm, the optimization design of the phase distribution of the diffractive-phase axicon can be achieved. The simulation calculations show that the new approach may successfully offer the design of the desired diffractive-phase axicon with long focal depth and high lateral resolution. A comparison of the performances of the holographic axicon with the phase-retardation functions from the geometrical optics prediction and the diffractive-phase axicon designed by the new approach is also presented.

Theory of Steady State Photoconductivity in Amorphous Semiconductors—Sensitization Due to the Presence of Different Types Recombination Centres

The present work attempts to connect Simmons and Taylors theory of steady state photoconductivity with Rose model of sensitization. A generalized formula of photoconductivity for a system with several recombination centres is given. An iteration procedure is suggested to solve the related rate equations. Using this formula and combining the particular energy band model of a-Si:H, the following result is obtained: when two types of recombination centres in the material exist and the electron capture coefficient for a centre of the second type is much smaller than that of the first type, and when the state density of the former is greater than that of the latter, the free electron lifetime will be greatly increased, therefore, the material will be sensitized. This theoretical result is consistent with that of the Rose model .The dependence of the photocurrent on temperature and incident photon flux are also given. Comparisons with the experimental results on hydrogenated amorphous silicon are made.

Calculation of Binding Energies of the Ground and Excited States of a Donor in GaAs-AlxGa1-xAs Step Quantum Wells*

In this paper we present for the first time completely analytical variational expressions for calculating the binding energies of the low-lying bound states of a hydrogenic donor in a quantum well (QW). These expressions can be used for the problem of the binding energy of an impurity in a general manner, e.g. for calculating the binding energies of a donor associated with subband states of any order in the QW with any arbitrary potential profile. To demonstrate the utilization of these expressions we have theoretically studied the binding energies for the ground and a few low-lying excited states of a hydrogenic donor in a stepped quantam well (SQW). The variations of the binding energies for these donor etates with impurity positions are investigated for different heights of the step potential. The numerical calculation results show that the binding-energy curves in the SQW, in contrast to the case of the flat quantum wells (FQWs), exhibit a non-symmetrical structure relative to the center of the QW and depend on the sub-barrier potential. The peak position in the binding energy curves shifts away from the region occupied by the sub-barrier as increasing sub-barrier height. The maximum in the binding energies increases as the step potential increaaes. These effects can be interpreted by referring to the variation of the electron probability-density distribution and of the lowest subband in the SQW. The shape of the binding-energy curves related to the 2p0like state is different from the other states. It is associated with the particular feature of the wave function of the 2p0 state in the QW.

Theoretical analysis of infrared stimulated current in a-Si:H

Abstract The infrared stimulated current (IRSC) in amorphous semiconductors is interpreted in terms of a model of two-step excited processes. The photoconductivity behavior with the excitation of two successive light beams, a visible light beam at an early time and an i.r. beam at a later time, is also analyzed based on the same model.

Effects of Extension and Overlap of Wavefunctions on Plasmon Modes in Symmetric Double-Quantum-Well Structures*

We investigate the effects of extension and overlap of wavefunctions on the dispersion relations of plasmon modes in symmetric double-quantum-well structures. We compare the approximate results in two-dimensional layer-gas (2DLG) model with the exact ones where the extension and overlap of the wavefunctions are included. Our numerical results show that the 2DLG model is a good approximation only in the wide barrier case in the long wavelength limit. When the barrier is thin, the extension and overlap of the wavefunctions cannot be neglected. We also show that the long wavelength gap of the inter-subband mode is proportional to the energy difference between the ground and the first excited energy levels.

Scattering and Diffraction of Electromagnetic Wave by 3D Periodic Structures with Subwavelength Scales:Self-consistent Field Approach*

We generalize the self-consistent field approach to deal with the problem of scattering and diffraction of electromagnetic wave by multiple obstacles. We derive in a rigorous way the relevant equations for determining the local field inside the scatterers as well as the far field of light scattering and diffraction for general periodic structures. To demonstrate the applicability of this approach, we calculate the far field distribution of the light scattering by a 3D grating. The convergent solution of the scattering field for this 3D grating can be evaluated numerically. From the comparison of the amplitudes of the diffraction factor in the 3D grating and an isolated scatterer, it is shown that the multi-scattering effect in the 3D grating cannot be neglected. The multi-scattering effect depends on the structural parameters of the 3D grating, the incident angle, and the polarization direction of the illuminating light.

Quantum percolation and ballistic conductance in a system of double-coupled chains

The quantum-mechanical calculation of electronic conductance in double-coupled chains as a function of the interchain bonding probability p is presented. The calculated results show that one still can see the basic plateaus in the ensemble-averaged conductance curves as a function of the Fermi energy for the weak disorder. In addition, dense irregularly oscillating structures are superimposed upon each plateau. The characteristics of the conductance are very sensitive to the presence of the interchain broken bonds. For the strong disorder (p ≈ 0.5) the conductance quantization breaks down. The accuracy of the quantization conductance rapidly drops down as the value of p approaches 0.5. The ensemble-averaged value of the logarithmic conductance as a function of the sample length exhibits a linear variation, determining a localization length. Both the localization length and the root-mean-square (RMS) value of the conductance fluctuations depend on p and the Fermi energy of electrons. The variations of the localization length and RMS with p are both of an approximate parabolic function around p ≈ 0.5. No percolation transition is found for this quasi-one-dimensional system, as expected.

INFLUENCE OF γ-RADIATION ON THE DIELECTRIC CHARACTERISTICS IN Rb 2 ZnCl 4 SINGLE CRYSTALS AT INCOMMENSURATE-COMMENSURATE PHASE TRANSITION

In this paper the influence of γ-radiation on the dielectric constants of Rb2ZnCl4 crystal at incommensurate-commensurate phase transition (hereafter abbreviated as INC-C transition) are studied. The thermal hysteresis occurs upon both cooling and heating runs, irrespective of whether the samples have been treated with γ-radiation or not. For the γ-irradiated sample, its transition point, Tc, between the INC and C phases is not changed, but the peak value of the dielectric constant at Tc increases abruptly, compared with that before γ-irradiation. When this sample is annealed at 40°C, the peak value restores to the incipient value for the sample free from γ-irradiation. The origin of the phenomenon of the thermal hysteresis of the dielectric constant may be due to the pinning effect of defects or impurities in the samples.

PROPERTIES OF PROPAGATION OF GUIDED ELECTRON WAVES IN COUPLED ASYMMETRIC QUANTUM WELLS

In this article we investigate the properties of the propagation of guided electron waves in the coupled asymmetric quantum wells. By decomposing the eigenfunctions of electron in the coupled double quantum wells in terms of the basis eigenfunctions of the individual wells, the expression of the mode amplitude functions for various bare states in wells has been given. By setting up appropriate boundary conditions one can simulate different injecting manners of electron into the system and study how the electron waves transfer among various states. The major electron wave transfer happens between the matching bare states in wells. The particular pattern of the transfer depends on the injecting manner and the energy of the incident electron. We also show the influence of the presence of the multimodes on the variation of the mode amplitude functions. It leads to producing some oscillations imposed on the original sinusoidal-like functions and causing incomplete transfer between a pair of states in channels due to the beating frequency effect or the interference among various modes.

KINETIC BEHAVIORS OF THE TIME-DEPENDENCE OF PHOTOCONDUCTIVITY IN A-SI-H

Abstract The behavior of the time dependence of photoconductivity have been calculated on the basis of a model for the kinetics of the defect creation process under the illumination in a-Si:H. Under the adiabatic approximation, the theory of the steady state photoconductivity together with spin generation rate equation is used to derive the time dependence of photoconductivity. The results are in agreement with experimental data.